(Received 16 May 1997)
5-Hydroxy-5a-cholest-8-en-3b-yl
acetate (8) (prepared in 6 steps starting from 7-dehydrocholesteryl
acetate (1) was transformed with ruthenium tetraoxide to 5-hydroxy-8,9-dioxo-8,9-seco-5a-cholestan-3b-yl
acetate (9). Treatment of the later 8,9-seco-5-hydroxy derivates with
lead tetraacetate(LAT) or mercuric oxide/iodine reagent (HgO/I2)
resulted, instead of the expected oxidative b-fragmentation of its
C(5)-C(10) bond, in the competing non-regio and non-stereoselective
acetoxylation of the a-positions next to the 8- and 9-oxo function (with LAT), and in
the ~ 82% recovery of the starting material(with HgO/I2). The
results are discussed in terms of the strong hydrogen bonding which exists
between the 5a-hydroxyl and the 8-oxo group.

(Received 16 May 1997)
In this paper an efficient synthetic approach to bridged cyclooctanone
framework is described, which is based on cyclization/alkoxy radical
fragmentation as a tactical combination of reactions. While the hypoidite
reaction of alcohol 11 proceeds in a normal fashion, the hypobromite
reaction of the same precursor unexpectedly affords dibromo derivative 14,
which, in turn, shows unusual reactivity in the presence of cesium acetate.

(Received 16 May 1997)
Using methyl 2,3,6-tri-O-methyl-a-D-glucopyranoside (12)
as the common starting material the synthesis of methyl 4-O-acetyl and
4-acetamido-4-deoxy-2,3,6-tri-O-methyl-a-D-gluco- (1 and 3,
respectively) and a-D-galactopyranosides (2 and 4, respectively) has been
described. Synthesis of 12 from methyl 4,6,-O-benzylidene-a-D-glucopyranoside,
via a series of regioselective blocking and deblocking reactions has
also been described.

(Receiver 8 July 1997)
The preparation of substituted steroidal A-ring 1,4-quinones by nucleophilic
addition results in pronounced regioselectivity at C(2), as confirmed by the
X-ray analysis of 2-methoxy-2,5(10)-diene-1,4,17-trione. Conformational
analysis (PM3, AMI, MNDO) of the suggested protonated intermediates and the
difference in the relative energies of the transition states are considered.
The results of cytotoxicity tests of selected quinones are also presented.

(Received 9 June 1997)
A series of 5-[2-N,N-di-n-propylamino)-ethyl]benzimidazole
derivatives has been synthesized by introducing different substituents into
position 2 of the molecule. The resulting ligands were checked in vitro
for their binding affinity at the dopamine (D1 and D2)
and serotonin (5-HT1A) receptors using synaptosomal membranes of the
bovine caudate nuclei and hippocampi, respectively, and appropriate
radioligands. None of the novel ligands expressed a binding affinity at the
dopamine D1 receptor. Compounds 1 and 10 were
competitors of both [1H]spiperone and [3H]-8-OH-DPAT.
Compound 3 was inactive in displacing [3H]spiperone form the dopamine D2
receptor, and compound 4 in displacing [3H]-8-OH-DPAT from
serotonin 5-HT1A receptor. The remaining seven novel ligands (2,5,6-9,11)
expressed a moderate mixed dopaminergic/serotonergic activity with the affinity
in the micromolar range of the concentrations. The fair affinity of compounds 11
and 8 produced by introducing very voluminous diphenylmethyl or
1-naphthylmethyl groups into position 2 of the parent compound, respectively,
indicates that both the D2 dopamine and 5-HT1A receptor
molecules are sterically very tolerant. The inactivity of ligands 3 and 10,
interpreted by the electroststic repulsive interaction of the positively
charged pyridyl groups and the above receptors, suggests that during design and
synthesis of dopamine and serotonin agonist and antagonists, electroni effects
should be aken into consideration.

(Received 23 June 1997)
Leakage of cell constituents from brewers years was induced by incubation with
thaumatin-like (TL) proteins isolated from barley grain. According to HPLC
analysis, the released mixture is characterized by a broad molecular mass
spectrum. It was partitioned by ultrafiltration into low- and high-molecular
weight (MW) portions (membrane cut-off 10 kDa) and characterized by HPLC,
SDS-PAGE and IEF. The low-MW portion consisted of eytoplasmic metabolites and
the high-MW of periplasmic glycoproteins with MW values, 33-55 kDa and pl 4-6.
Comparative experiments were performed with amphiphilic compounds which
revealed a consistence of leakage effects with increasing hydrophobicity.
Examination of the leaked cell constituents induced by incubation of yeast
cells with dodecanoate and glycodeoxycholate sodium salts showed that the
released mixtures were composed of cytoplasmic low-MW metabolites and high-MW
nucleic acids. This suggests that thydrophobic molecules, as well as
hydrophobic domains in TL-molecules generally determine the transmembrane
transport from the cytoplasm while the basic groups in the TL-molecules
participate in a specific detachment of periplasmic proteins.

(Received 24 July 1997)
Tissue engineering is a new interdisciplinary field which combines the
theoretical principles and practical approaches of biology, polymer chemistry
and chemical engineering to create functional tissue substitutes for scientific
studies and clinical applications. Isolated cells are stimulated to remodel
their parent tissues in vitro by using biodegradable polymer scaffolds
which provide a temporary 3-dimensional structure for tissue growth, and
bioreactors which provide control over physiological parameters and
hydrodynamic forces in the cell microenvironment. Highly porous polyglycolic
acid scaffolds were specifically designed for tissue engineering and tested
with respect to their biocompatibility, structure and degradation rate. Various
tissue engineering bioreactors including static, well mixed and rotating
vessels have been used in cultivations of natural and engineered tissues for up
to 3 months. This paper reviews the use of cells, polymers and bioreactors to
engineer cartilage and other tissues, and study tissue morphogenesis under controlled
in vitro conditions. Some recent findings on the effects of bioreactor
design and operating conditions on the structure, metabolic and biomechanical
function of the growing tissue are also discussed.

(Received 17 April 1997)
The concept of response reactions, elaborated in full detail in a few recently
published papers, is interpreted in an alternative way, in order to make its
chemical meaning easier to understand, and to visualize its most important
features. The reaction extents in multiple-equilibrium systems are then
interpreted in terms of response reactions. A geometric representation of the
response reactions in three-component multiple-equilibrium systems is presented
and a few illustrative examples provided.

(Received 23 April 1997)
The singlet and triplet electronic states of the B2H2
radical are calculated by means of the quantum chemical ab initio
method. The spectrum is compared with those of related molecular and atomic
species. It is found that there is a great similarity between the global
structures of the spectra of B2H2 and O2. The
nature of the binding in B2H2 is discussed in terms of
the composition of the molecular orbitals and the molecular geometry dependence
of their energy.

(Received 18 June 1997)
The template reaction of warm alcoholic solutions of Mn(OAc)2, S-methylisothiosemicarbazidehydrogen-iodide
and 2,6-diacetylpyridine, in the presence of an excess of NH4NCS
yielded complexes of the general formula [Mn(NCS)(H2L)ROH]NCS, where
H2L denotes the pentadentate N5 ligand
2,6-diacetylpyridine bis(S-methylisothiosemicarbazone), and R =
Me, Et. X-ray analysis of the MeOH complex showed its pentagonal-bypyramidal
configuration (PBP), with the H2L ligand in the equatorial plane,
while the axial positions were occupied by one NCS group and a MeOH molecule.
On the basis of IR spectra similar structure is supposed for the EtOH complex.
the desolvate complex [Mn(NCS)2(H2L)], probably has a PBP
structure too, in which both the axial positions are occupied by a NCS groups.

Compounds
analogous to intermediates in hydrolytic cleavage of peptides by palladium(II)
complexes

TATJAN
N. PARAC and NENAD KOSTIC

Department
of Chemistry, Iowa State University, Ames, IA 50011, USA

(Received 19 May 1997)
Reactions of cis-[Pd(en)(sol)2]2+, in which en is ethylenediamine
and sol is water or acetone, with dipeptide N-CBZ-Met-Gly-OEt(1)
and its thioamide (symbol t) analog N-CBZ-Mett-Gly-OEt(2)
are studied by 1H- and 13C-NMR spectroscopy. The compound
1 reacts with cis-[Pd(en)(sol)2]2+ forming
the complex cis-[Pd(en)(N-CBZ-Met-Gly-OEt)(sol)]2+ (3),
in which the peptide coordinates to palladium(II) via the sulfur atom in the
side chain of methionine. This mode of coordination is favorable for the
hydrolysis of the Met-Gly amide bond. The compound 2 reacts with cis-[Pd(en)(sol)2]2+
and forms the complex cis-[Pd(N-CBZ-Met-Gly-OEt)(sol)2]2+
(4), in which the thiopeptide coordinates to palladium(II) via
two sulfur atoms, those in the methionine side chain and in the thioamide
group. The bidentate coordination in 4 is unfavorable for the hydrolysis
of the thioamide group, because the binding of thiocarbonyl group to
palladium(II) results in the strengthening of the thioamide C-N bond.
Therefore, the complex 4 is an unreactive analog of the reactive
intermediate in the cleavage by external attack of solvent molecule. These
studies with model complexes contribute to the understanding of the mechanism
by which palladium(II) complexes hydrolyze peptides and proteins.

(Received 3 July 1997)
We propose to use the temperature dependence of laboratory frame
cross-relaxation rate to deduce the absolute value of interproton distances in
solutions. For rigid molecules undergoing random motion, the cross relaxation
rate becomes zero at a characteristic temperature T0, while at a
higher temperature Tmax it reaches its maximum. At Tmax
the correlation time is precisely defines and the interproton distance can be
calculated directly from the cross-relaxation rate. The applicability of the
method is tested on a small model compound, cyclo (L-Pro-Gly). For the geminal
proton pair of glycin, the measured distance of 1.89±0.011A disagrees with the
model value beyond the experimental error, indicating that the accepted model
parameters need to be reevaluated.

Key words: NMR,
cross-relaxation, distances, cyclo-(L-Pro-Gly)

J. Serb. Chem. Soc. 62(9) 873-884 (1997)

JSCS-2455
UDK 546.923:541.183:66.094 Original scientific paper

02
adsorption and reduction on Pt(111) modified with Br and Ag adlayers. an
electrochemical and in situ X-ray diffraction study

(Received 17 July 1997)
Structures of Br and Ag adlayers were determined on the Pt(111) in the absence
and in the presence of O2 reduction. On Pt(111), the commensurate
(3x3) Br adlayer was found between 0.3 and 0.8 V in HClO4 solutions.
O2 reduction is blocked by the (3x3) Br adlayer up to 0.5 V. At this
potential, the ordered phase vanishes. Adsorbed disordered Br ions change the
mechanism of O2 reduction on Pt(111) from a 4e- into a 2e-reaction.
Ag forms a hexagonal incommensurate bilayer at underpotentials while the first
monolayer has a commensurate (1x1) structure. A complete inhibition of O2
reduction on Pt(111) has been observed upon deposition of Ag monolayer.
Analysis of the inhibition of O2 reduction as a function of the Ag
coverage shows that during reduction O2 adsorbs in a bridge
configuration on Pt(111).

(Received 11 March 1997)
Methanol oxidation on platinum low-index surfaces was carried out in sodium bicarbonate
solutions. It was shown that the low-index planes of platinum adsorb OH-
and bicarbonate anions, as well as "poisoning species". The surface
activity was directly correlated to the OHad species coverage. The
"poisoning species", produced during methanol oxidation, participate
in the reaction at higher potentials, but partially block the surface at lower
potentials. A dual path reaction mechanism was proposed based on the
assumptions that HCO is a reactive intermediate and that formate is a reaction
product in the main path, while CO2 is a product of "poisoning
species" oxidation in a parallel reaction path.

It has been established that
the two rules formulated for thin-layer salting-out chromatography - RM
pair linearity rule and Proportionality rule - can also be applied
to C18 column chromatography in all the cases where the Bosch model
is valid, but instead of RM values, the corresponding log k values
are used. In addition, it has also been found that he obtained RPP values
designate a new polarity scale for solvent systems.

(Received 9 June 1997)
The need for the removal of mercury from the precious metals extraction circuits
may be influenced by process, environmental, as well as by occupational hazard
reasons. In these circuits mercury is mainly present as cyanide complexes. The
effectiveness of any stabilization method will be influenced by the high
thermodynamic stability mercury-cyano species exhibit in an aqueous solution.
In this study, we have investigated the utility of the trisodium salt of
trimercapto-s-triazine (TMT, CAS No. 17766-26-6) for precipitation of
mercury from the corresponding cyano-complexes. The influence of pH, mole ratio
of TMT to Hg, coagulant and flocculant additions, excess of free cyanide, as
well as the presence of other metal-cyano species on the effectiveness of
mercury precipitation were investigated in detail. Also, a "real world"
precisious metals mill effluent initially containint ~ 7 µg/mL (ppm) of Hg was
treated and it was found that greater than 99.8% of the mercury was removed
from the solution as an insoluble Hg/TMT complex.